An email automation platform is a software solution that enables businesses and marketers to automate and streamline their email marketing campaigns and communication with subscribers or customers. These platforms offer a range of features to create, schedule, and personalize email messages, as well as automate various aspects of email marketing. Here are some popular email automation platforms as of my last knowledge update in September 2021:

1. Mailchimp: Mailchimp is known for its user-friendly interface and offers email marketing automation, drip campaigns, segmentation, and detailed analytics. It's suitable for businesses of all sizes.
   
2. Constant Contact: Constant Contact provides email marketing automation tools, including automation workflows and triggered emails, along with other email marketing features. It's often email automation platform used by small businesses and nonprofits.
   
3. AWeber: AWeber offers email automation features for creating automated email sequences and autoresponders. It's suitable for small to medium-sized businesses.
   
4. GetResponse: GetResponse provides marketing automation tools, including email automation, workflow automation, and marketing funnel creation. It's suitable for businesses looking for an all-in-one marketing platform.
   
5. ActiveCampaign: ActiveCampaign is known for its advanced marketing automation and CRM features. It's a powerful platform for creating highly personalized and automated email campaigns.
   
6. HubSpot: HubSpot's marketing automation platform includes email marketing automation, lead nurturing, and CRM integration. It's ideal for businesses focused on inbound marketing strategies.
   
7. ConvertKit: ConvertKit is designed for content creators, bloggers, and online businesses. It simplifies email automation and segmentation for targeted communication.
   
8. Drip: Drip is an eCommerce-focused email marketing and automation platform with advanced automation capabilities. It's tailored to help online retailers create personalized customer journeys.
   
9. SendinBlue: SendinBlue provides email marketing automation, transactional emails, and SMS marketing. It's known for its affordability and is suitable for cost-conscious businesses.
   
10. MailerLite: MailerLite offers email marketing automation features, including workflow automation and personalization. It's known for its simplicity and affordability.
    
11. Campaign Monitor: Campaign Monitor includes marketing automation tools for creating automated customer journeys and personalized email campaigns. It's often chosen by agencies and larger businesses.
    
12. SharpSpring: SharpSpring offers marketing automation, CRM integration, and email marketing automation features. It's suitable for businesses looking for an all-in-one marketing solution.
    
13. Salesforce Marketing Cloud: Salesforce Marketing Cloud provides advanced email marketing automation, CRM integration, and customer journey management. It's suitable for larger enterprises.